In oil and gas equipment, some may employ seals that are pressure energized; that is, fluid pressure on one side of the seal causes the sealing surfaces into greater contact with each other. For example, fluid pressure applied to the recessed or concave side of an annular cup-shaped or lip seal increases the sealing force. A lip seal does not seal against fluid pressure applied to convex side, thus it is a one-way pressure energized seal.
Many bi-directional seals are not pressure-energized. They seal due to deformation against the seal surfaces applied by setting forces during installation. They seal in both directions independently of the fluid pressure. For example, casing hanger seals in wellheads typically are not pressure energized. Often they are made of metal and permanently deformed.
An advantage to a pressure-energized seal is that it does not require a high setting force to be applied before being exposed to fluid pressure. For example, a subsea swivel connector has tubular members with aligned bores. Each tubular member has an external flange with a bolt pattern for bolting to pieces of subsea equipment. To install, the swivel connector is positioned between the subsea equipment and bolted to one piece. Then the unbolted flange is rotated until aligned with the other piece to allow the other piece to be bolted. Once bolted to both pieces, no further swivel movements are needed. A metal-to-metal seal between the two parts of the swivel would prevent the parts from rotating if it is of a permanently deformed type. Elastomeric seals between the two parts of the swivel work, however, they do not have the longevity of metal-to-metal seals.
A one-way pressure-energized metal seal between the two parts of the swivel could be employed, but has disadvantages. The internal fluid pressure in the swivel to energize the seat would occur after both parts of the swivel are made up with the two pieces of subsea equipment. Pressure-energized deformation of the metal seal would normally be elastic and below the yield strength of the metal of the lip seal. When the internal fluid pressure is removed or reduced, the contact forces between the sealing surfaces diminish. A reduction in contact forces may result in leakage of sea water from the exterior to the interior, which is detrimental. There are other applications for a two-way pressure-energized metal seal.